How to Resolve the 'UnknownHostKey' Error When Using JSch's SFTP Library?

Resolving Java UnknownHostKey Issue with JSch SFTP Library

When using the Java SFTP library (JSch), you may encounter the "UnknownHostKey" error due to strict host key checking. This error is raised because JSch cannot verify the authenticity of the host key presented by the remote server.

Root Cause:
The error occurs when you attempt to connect to a remote SFTP server for the first time. JSch performs host key checking to ensure that you are connecting to the correct server and not to a malicious imposter.

Solution:
There are two common approaches to resolve this issue:

1. Skipping Host Key Checking:
This method is not recommended, as it compromises the security of your connection. However, if you are certain that the host key is trusted, you can disable host key checking by setting the "StrictHostKeyChecking" property to "no" before connecting to the server:

java.util.Properties config = new java.util.Properties();
config.put("StrictHostKeyChecking", "no");
session.setConfig(config);

2. Adding Expected Host Key:
For enhanced security, it is advisable to add the expected host key to JSch's host key repository. This can be achieved using one of the following methods:

Generate Known Hosts File:

• Execute the following command from a *nix server:

  ssh-keyscan example.com > known_hosts

• Place the generated "known_hosts" file in a location accessible to JSch.

• Use "JSch.setKnownHosts(path)" to point to the file:

  JSch.setKnownHosts("path/to/known_hosts");

Add Host Key Manually:

• Retrieve the host key from the server using SSH tools (e.g., "ssh-keyscan").
• Create a JSch "HostKey" instance using the retrieved key.

• Add the HostKey to the host key repository:

  JSch jsch = new JSch();
  HostKey hostKey = jsch.getHostKeyRepository().add(hostname, publicKey, algName);

Additional Note:

Ensure that the host key you provide matches the actual key presented by the remote server. If the keys do not match, the connection will still fail due to a security exception.

The above is the detailed content of How to Resolve the 'UnknownHostKey' Error When Using JSch's SFTP Library?. For more information, please follow other related articles on the PHP Chinese website!